Host responses to the 13762 mammary adenocarcinoma growing as an ascites tumor in the syngeneic Fischer rat are being studied. Lymphocytes specifically cytotoxic to the tumor appear in the spleen in cycles, about day 6-8 and again, 13 days after tumor inoculation. Spleen cells obtained 9-13 days after inoculation lack cytolytic activity and specifically inhibit in vitro generation of cytolytic lymphocytes. Serologic blocking factors are detected in tumor bearing rats 4 days after injection of 10 to the 4th power tumor cells; at day 12, blocking factors are located in the IgG- (160,000 daltons) and low-molecular weight (less than 70,000 daltons) fractions; the latter fraction is presumably solubilized tumor antigen. The IgG-blocking factor appears to be an immunoglobulin directed against receptors on the cytotoxic T cell (putative anti-receptor antibody). By day 20, humoral blocking activity also appears in the high molecular weight fraction (less than 200,000) and is presumed to be tumor antigen-antibody complex. Blocking and suppression of the day 7 and day 13 cytotoxic cells appears to be mediated by two separate sets of IgG blocking factors and suppressor cells; day 20 blocking factor and day 20 suppressor cells are more efficient on day 13 cytotoxic cells than on day 7 cytotoxic cells. We are attempting to develop stable lines of cytotoxic cells derived from spleens of animals inoculated with tumor 7, 13 and 20 days earlier, using helper T-cell derived growth factor. If these are obtained, we will have reagents enabling us to dissect out differences in specificity in surface membrane receptors for the tumor antigen(s).